Fluidized bed work involves a gas-solid contacting process in which a bed of particles is lifted, agitated and thus enlarged by rising streams of gas. The amount of lifting and enlargement relates to the gas velocity, the size and weight of the particles and the density of their dispersion throughout the gas. For coating work the particles are typically fine, light in weight and densely dispersed, the gas velocity is low and the bed is enlarged to accommodate the size of article to be coated. At low velocities utilized in coating work, the bed behaves as a slowly boiling liquid, hence the term "fluidized bed."
Perhaps the most pervasive problem in this art is to obtain and maintain good agitation and uniform dispersion of particles throughout a work chamber enclosing a fluidized bed. Nearly all beds are supported on a substantially horizontal member which is also utilized as a gas diffuser. A compressed gas is introduced under and diffused through the member and the bed to create a dense cloud of gas-borne particles above the member. A problem is that large bubbles of gas are created which rise through the bed and erupt on the top surface like bubbles in a boiling liquid. Such vigorous bubbling is to be corrected because it causes uneven coating and such correction is typically achieved by vibrating the bed. When satisfactory vibration is accomplished, uniformly sized pores appear between particles which may align somewhat uniformly into fine channels for gas to move upwardly. Unfortunately, when obstructions are incurred by the gas, undesirable turbulence may ensue so chamber sidewalls are preferably made smooth and vertical, especially for difficult work.
The bed conditions obtained by conventional means are adequate for most coating work. For example, articles are heated and then immersed in such beds and corrosion resistant polymers are deposited on the articles in coatings of spark resistant, satisfactory thickness. However, many articles have irregular features including lateral indentations which require lateral mobility (usually horizontal) of particles for proper coating. Typical articles requiring horizontal mobility are printed wiring board substrates which may have hundreds of tiny through-bores which are to be uniformly, internally coated. Coating such substrates is a challenging task.
Other problems are present in fluidized beds which are vibrated by conventional means. For example, thicknesses of coatings can be expected to range from about .+-.4 to .+-.5 mils of a given value whereas .+-.11/2 mil is a desired tolerance. High overall bed densities are difficult to achieve due to gradients in density over such beds making them inefficient in coating articles. In-process addition of particles often disturbs conventional bed operation. It is believed that many of these problems are at least mitigated by improving horizontal mobility of particles in a bed. It is further believed that such mobility is often discouraged by inefficient and non-uniform vibration of fluidized beds.
Vibration has been achieved by mechanical cranks, electro-magnetic oscillators, and by pneumo-oscillators (pulsing air mechanisms). These expedients have been applied to the bed support member to the walls of the work chamber or both. Sometimes they have been applied to work chambers having portions of the walls made of elastic material to assist or magnify applied vibrations. When vibrators have been applied directly to sidewalls, the intention and the effect has been to vibrate the gas and gas-borne particles disposed between the sidewalls.
Some of the above expedients impart forces having horizontal components which assist in driving particles into indentations in articles. However, such forces are seen to be heretofore applied in a non-uniform manner between and along opposing sidewalls. Also, the horizontal components of forces have been relatively uncontrolled in causing particle movement. For example, lateral excursion of particles often have been too long, causing turbulence in some parts of a bed, and too short to penetrate bores in other parts of the same bed.
Accordingly, it is desirable to develop new and improved expedients for providing horizontal mobility of particles in a fluidized bed. Such mobility should be provided without creating large bubbles and turbulence. It is further desirable to impart to particles, horizontal mobility which is uniformly distributed over and between opposing sidewalls. Such mobility should be adjustably limited to horizontal particle excursions required by the work to avoid turbulence in a bed.